The present invention relates, in general, to the pulp and paper industry, and in particular, to a new and useful apparatus, system and method for on-line, in-situ monitoring and/or controlling the concentration of lignin in wood pulp and black liquor.
In the pulp and paper industry, the production of paper products requires that lignin be partially removed from the wood chip feed stock prior to making paper products. Lignin is a polymer of complex chemical structure which "cements" the wood's cellulose fibers together. The process by which lignin is removed is referred to as delignification. The most prevalent method of delignification is by chemical means in which raw wood chips and chemicals are combined at controlled pressure and temperature in a vessel known as a digester. While in the digester, the amount of lignin removed from the wood chips determines the product quality, the product yield, the amount of energy consumed, the quantity of chemicals consumed, and the product cost. Fluid drained from the digester during delignification contains lignin removed from the wood chips and is referred to as "black liquor". The black liquor is used as fuel to the boiler to produce process steam.
The measurement of the residual lignin remaining in the pulp, which exits the digester, is most commonly carried out by laboratory analysis of hourly samples of the digester output (samples are typically obtained at the last stage of the brownstock washer). The lab analysis takes approximately one hour and therefore is a poor method for providing process control feedback and cannot be used for feedforward control. This lab analysis is a back titration method which measures the consumption of potassium permanganate and is only an approximation to the lignin concentration. The output of the titration analysis is referred to as a "KAPPA Number" and the procedure is documented in TAPPI procedure T236 cm-85, "KAPPA Number of Pulp". A number of manufacturers have produced automatic sampling and titration devices which have been tried in pulp mill situations but they have been mostly unsuccessful in providing accurate long-term results and did not eliminate the hour delay between the process and measurement of residual lignin.
The ultraviolet absorption and fluorescence properties of lignin have long been known and a number of researchers have reported results of measurements in solutions containing lignin. Both the absorption techniques (e.g. see Kleinert, T. N. and Joyce, C. S., "Short Wavelength Ultraviolet Absorption of Various Lignins and Related Substances," Part I, Pulp and Paper Magazine Can. 58, No. 5, April 1957, pp. 154-158.; Joyce, C. S. and Kleinert, T. N., Ibid., Part II, Pulp and Paper Magazine Can. 58, No. 6, May 1957, pp. 131-148.; Kleinert T. N. and Joyce, C. S., Ibid., Part III, Pulp and Paper Magazine Can. 58, No. 7, June 1957, pp. 215-219; Kleinert T. N. and Joyce, C. S., Ibid., Part IV, Pulp and Paper Magazine Can. 58, October 1957, pp 147-152; Hartler, N. and Norrstrom, H., "Light Absorbing Properties of Pulp and Pulp Components," TAPPI Journal, Vol. 52, No. 9, September 1969; Norrstrom, B. and Teder, A., "Absorption Bands in Electronic Spectra of Lignins, Part 2, Band Intensities for Alkali Lignins from Spruce," Svensk Papperstidning, 15 Jun. 1971; and Sjostrom, E. and Haglund, P., "Spectrophotometric Determination of the Dissolution of Lignin During Sulfite Cooling," TAPPI Journal, Vol. 47, No. 5, May 1964, pp 286-291) and the fluorescence techniques (e.g. see Bublitz, W. J., "Fluorescence of Pulping Liquors: A Tool for Digester Control?", pp 73-76; Baumgartner, D. J., Feldman, M. H., and Gibbons, C. L., "A Procedure for Tracing Kraft Mill Effluent From an Ocean by Constituent Fluorescence," Water Research, Pergamom Press, Vol. 4, 1971, pp 533-544; Bublitz, W. J. and Wade, D. C., "Applied Waste Liquor Fluorescence to Control Pulp Quality," Svensk Paploerstidnin, No. 18, 1979, pp 535-538; Wilander, A., Kvarnas, H. and Lindell, T., "A Modified Fluorometric Method for Measurement of Lignin Sulfonates and Its In-Situ Application in Natural Waters," Water Research, Vol. 8, 1974, pp 1037-1045; and Demas, J. N., Excited State Lifetime Measurements, Academic Press, New York, 1983, pp 53-58), have all been applied to very dilute solutions.
The fluorescence techniques have been used primarily as a method of detecting trace quantities in effluent streams. All of these approaches made use of very dilute lignin solutions where the absorption and fluorescence signal are linearly related to lignin concentration. The concentration of lignin in these solutions is typically 2000-10,000 times more dilute than the concentration of lignin in "black liquor" found in the pulping process. Thus, use of these techniques requires precise sample preparation prior to measurement. A number of devices which attempt to monitor the lignin concentration in "black liquor" during the pulping process by UV absorption techniques (alone or in combination with chemical analysis) have been produced. See Tikka, P. O., and Virkoka, N. E., "A New Kraft Pulping Analyzer for Monitoring Organic and Inorganic Substances," TAPPI Journal, June 1966, pp 66-71; Williams, D. J., "The Application of Ultra-Violet Absorption Characteristic of Lignin to the Control of Pulp Uniformity," Appita, Vol. 22, No. 2, September 1968, pp 45-52; and Capart, R., Obese-Jecty, K., Le Cardinal, G., and Gelus, M., "Contribution to the On-Line Kraft Pulping Control," PRP 4 Proceedings, Ghent, 1980, pp 121-128. These devices require sample preparation and dilution prior to measurement and are therefore not in-situ, not real-time, and introduce sampling and dilution errors.
Use of ultraviolet absorption has recently been extended to the measurement of residual lignin in wood pulp (see Kubulnieks, E., Lundqvist, S., and Pettersson, T., "The STFI OPTI-Kappa Analyzer, Applications and Accuracy," TAPPI Journal, November 1987, pp 38-42). This device is marketed by Asea Brown Boveri under the trade name "Opti-Kappa Analyzer". In this approach, the pulp stream is sampled approximately once every 5 minutes. The pulp sample is screened, washed thoroughly, and diluted significantly. The diluted sample is circulated in a loop where UV light absorption is measured over a prescribed time period and the pulp concentration in the slurry, i.e. pulp consistency, is measured independently. This system involves sampling error, screening error, and pulp consistency measurement error. Although the system provides results much faster than the conventional lab titration process, it is still off-line. The washing requirements of this device are stringent since any small amount of black liquor remaining in the diluted solution will absorb UV light and produce error. Bannier Technology Group (BTG, Inc.) also offers a device which operates on a similar principle but uses UV reflection rather than absorption. The BTG device is marketed under the name "KNA-5000 Kappa Number Analyzer".
All of the investigations and devices discussed so far used broadband lamps as the source of UV light. In 1986, researchers at the National Bureau of Standards (see Horvath, J. J., Semerjian, H. G., "Laser Excited Fluorescence Studies of Black Liquor," Proceedings of the SPIE, Vol. 665, June 1986, pp 258-264) performed fluorescence tests on diluted black liquor samples using a laser as the source of UV light. Although their investigation resulted in better signal to noise ratios, they essentially did not extend the art beyond that of previous investigators. They were only able to obtain a functional relationship between fluorescence and lignin concentration in very dilute samples of black liquor (less than 1300 ppm, which is orders of magnitude less than the in-situ concentrations) and did not investigate pulp at all. They did not provide any insight into how one might be able to use either UV absorption or fluorescence techniques to extend the useful measurement range beyond the highly diluted state.
They did mention that this process was a candidate for in-situ monitoring but provided no rational explanation of how the dilution requirement could be overcome. They did mention that the measurement could be made more acceptable for field use by using optical fibers to guide the UV excitation light to the process stream and carry the fluorescence signal back to the opto-electronics unit.